1. New Technical Field
This invention relates to data modulation circuitry and, more particularly, to a binary phase shift key modulator with programmable output level control.
2. Description of the Prior Art
The development of cable television systems has reached the stage where not only is the provision of two way information flow desirable but is practically required by the implementation of new services. For example, in the implementation of impulse pay-per-view service where the subscriber may impulsively select an event for viewing and assume a charge, at least one data channel such as a telephone communication channel or an RF channel is required in an upstream (reverse) direction from a cable television subscriber to a cable television headend to report service usage data. Other uses for a return path include power meter reading, alarm services, subscriber polling and voting, collecting subscriber viewing statistics, and home shopping. While not every cable television system operator provides for two way transmission, manufacturers of cable television equipment have tended to provide for upstream transmission in the direction from the subscriber toward the headend. Practically all such manufacturers provide so-called split or two way systems having a spectrum of frequencies for upstream transmission which at least includes a band from 5 to 30 megahertz. This band of interest comprises cable television channel T7 (5.75-11.75 megahertz), T8 (11.75-17.75 megahertz), T9 (17.75-23.75 megahertz) and T10 (23.75-29.75 megahertz). These return path channels, each having television signal bandwidth, may be used, for example, for video conferencing. Whether a so-called "sub-split", "mid-split" or "high-split" system is applied for two way transmission by a headend operator, all three types of split transmission systems typically involve an upstream transmission in the 5-30 megahertz band of interest.
In articles published at the 1987 National Cable Television Conference and in U.S. Pat. No. 4,586,078, Richard Citta and Dennis Mutzbaugh propose that a 45 kilobit data signal may be alternately transmitted by a coherent phase shift keying (CPSK) technique over carriers at 5.5 megahertz and 11.0 megahertz or in the vicinity of the T7 and T8 cable television channels respectively. A switch at the subscriber terminal alternately selects the 5.5 MHz carrier or the harmonically related 11 MHz carrier for transmission. This form of alternating carrier transmission of messages is continued until the data is successfully received. In other words, alternating transmission on the two carriers occurs until an acknowledgment signal indicating successful receipt of a message is received at a terminal. While the choice of these carrier frequencies is claimed to avoid the noise distribution peaks caused by interference noise, there is considerable concern that such a modulated phase shift keyed data stream will run into noise peaks in cable television distribution network outside of the investigations of Citta et al. Referring to FIG. 2 republished here from U.S. allowed application Ser. No. 07/188,478 filed Apr. 29, 1988, (issued as U.S. Pat. No. 4,912,721) transmission at 5.5 MHz should be practically impossible. Noise peaks have been known to appear and disappear based on time-of-day, season, and other considerations.
Other return path or upstream data transmission schemes have been tried. These schemes include, for example, the telephone system, described as "ubiquitous" by Citta et al. In other words, the return data path to a cable television headend is not provided over the cable television distribution plant at all. The serving cable is intentionally avoided either because of the interference noise problem in a split system or because the system is a one way downstream system. Instead, the subscriber's telephone line is used for data transmission. In this instance, however, there is concern that local telephone data tariffs may require the payment of the line conditioning surcharges if the telephone line to a subscriber's home is used for data transmission in addition to normal "plain old" telephone service. Furthermore, the telephone line is only available when the subscriber is not using it, requiring an unscheduled or periodic data flow.
Another known return data transmission scheme involves the application of a separate data channel at a carrier frequency that avoids the troublesome 5-30 megahertz band. This scheme, of avoiding the noisy 5-30 megahertz band, is only possible in midsplit and high split systems.
So-called spread spectrum transmission of data is a technology which evolved for military requirements from the need to communicate with underwater submarines in a secure manner. Spread spectrum derives its name from spreading a data signal having a comparatively narrow bandwidth over a much larger spectrum than would be normally required for transmitting the narrow band data signal.
U.S. Pat. No. 4,635,274 to Kabota et al. describes a bidirectional digital signal communication system in which spread spectrum transmission is applied for upstream data transmission in a cable television system. Such technology is very expensive, however, when compared with telephone data return.
Consequently, despite the development of spread spectrum and other RF data return, the requirement remains in the cable television art for an upstream data transmission having high data throughput from a plurality of subscriber premises to a cable television headend utilizing the cable television distribution plant and which is relatively impervious to interference noise.